The Role of Platelet and its Interaction with Aspirin

Rev. Fac. Med. 2016 Vol. 64 No. 2: 351-63
351
R E VIE W ART I C LE
DOI: http://dx.doi.org/10.15446/revfacmed.v64n2.53789
The Role of Platelet and its Interaction with Aspirin
El rol de las plaquetas y su interacción con la aspirina
Luisa Fernanda Zúñiga-Cerón1• Janh Sebastián Saavedra-Torres1• Carlos Alberto Navia-Amézquita2
Received: 26/10/2015
Accepted: 18/01/2016
1
Universidad de Cauca - School of Health Sciences - Laboratorio al Campo - Popayán - Colombia
2
Universidad de Cauca - School of Health Sciences - Department of Physiological Sciences - Popayán - Colombia
Corresponding Author: Luisa Fernanda Zúñiga-Cerón. Calle 5N No. 6A-116, edificio D, apartamento 404. Phone:
+57 2 8235352. Popayán. Colombia. E-mail: lfzuniga@unicauca.edu.co.
| Abstract |
Introduction. Platelets are not strictly “cells”, they are
cytoplasmic fragments that are produced by the division of
the cytoplasm of the megakaryocytes in the bone marrow;
among its properties of adhesion and aggregation, platelets
are involved in primary hemostasis, thrombosis, repair and
tissue regeneration processes. To modify the pro-thrombotic
platelet properties aspirin (ASA) is provided, which allows
blocking the synthesis of prostaglandins in order to irreversibly
inhibit the COX1 and prevent platelet aggregation. The current
guidelines govern the use of aspirin in the prevention of
cardiovascular diseases.
Objective. In this review paper graphics were created to
facilitate the understanding of platelets and its interaction with
various physiological and pathological processes.
Methodology. A literature review of a total of 74 documents
such as articles and books was carried out; it was obtained
from bibliographic searches in the following databases:
Scielo, NEJM, Elsevier, Pubmed, Redalyc, Wiley, Springer,
ScienceDirect, BVS, Nature Riviews, EBSCO, Naxos
(databases offered by the University of Cauca). Mendeley free
access program was used for the management and organization
of information.
Zúñiga-Cerón LF, Saavedra-Torres JS, Navia-Amézquita CA. The Role of
Platelet and its Interaction with Aspirin. Rev. Fac. Med. 2016;64(2):351-63.
English. doi: http://dx.doi.org/10.15446/revfacmed.v64n2.53789.
Resumen
Introducción. Las plaquetas no son estrictamente células, son
fragmentos citoplasmáticos que se originan por la división del
citoplasma de los megacariocitos en la médula ósea; entre sus
propiedades de adhesión y agregación, las plaquetas participan
en procesos de hemostasia primaria, trombosis, reparación
y regeneración de tejidos. Para modificar las propiedades
protrombóticas de las plaquetas, se administra aspirina (ASA),
la cual permite bloquear la síntesis de prostaglandinas para
inhibir irreversiblemente la COX1 y para prevenir la agregación
plaquetaria. Las directrices actuales determinan el uso de la
aspirina para la prevención de las enfermedades cardiovasculares.
Objetivo. Crear gráficos que faciliten la comprensión de las
plaquetas y su interacción con diversos procesos fisiológicos
y patológicos.
Conclusion. It is recognized the role of platelet in different
physiopathological processes and thus its interaction with
aspirin, preventing its aggregation and thrombus formation
in the spleen and other organs, this way contributing to the
prevention of future cardiovascular events.
Metodología. Se desarrolló una revisión bibliográfica de un
total de 74 documentos como artículos y libros obtenidos
a partir de la búsqueda bibliográfica de las bases de datos
Scielo, NEJM, Elsevier, Pubmed, Redalyc, Wiley, Springer,
ScienceDirect, Medwave, BVS, Nature Riviews, EBSCO,
Naxos —Bases de datos que ofrece la Universidad del
Cauca—. Para la gestión y organización de la información se
utilizó el programa Mendeley de libre acceso.
Key Words: Platelet; Aspirin; Coagulation; Growth Factors;
Regeneration; Anticoagulant (MeSH).
Conclusión. Se reconoce el rol de la plaqueta en diversos
procesos fisiopatológicos y de esta manera la interacción con
352
la aspirina, evitando su agregación y la formación de trombos
en el bazo y otros órganos. Se contribuye de esta manera en
la prevención de futuros eventos cardiovasculares.
Palabras claves: Plaquetas; Aspirina; Coagulación; Factores
de crecimiento; Regeneración; Anticoagulantes (DeCS).
Zúñiga-Cerón LF, Saavedra-Torres JS, Navia-Amézquita CA. [El rol de
las plaquetas y su interacción con la aspirina]. Rev. Fac. Med. 2016;64(2):351-
The Role of Platelet and its Interaction with Aspirin: 351-63
the bone marrow (7) (Table 1), which are derivative from
the pluripotent hematopoietic stem cell (PHSC) (8); which is
activated by colony-stimulating factors that are synthesized
into cells such as macrophages, fibroblasts and endothelial
cells (4,6,9,10).
Table 1. Anatomy of the platelet.
Anatomy of platelet
For example: anatomy
63. English. doi: http://dx.doi.org/10.15446/revfacmed.v64n2.53789.
Introduction
The description of particles in blood smaller than
leukocytes and erithrocytes dates from the end of the
eighteenth century. However, it was only between 1865
and 1877 that these corpuscles were clearly described,
but without understanding their origin, significance and
function (1).
There is a general agreement that Giulio Bizzozzero was
the first, in the years 1881-1882, to establish their central role,
not only in physiological haemostasis, but also in thrombosis.
It is noteworthy that the discovery of blood platelets and
their function took place when the knowledge about the
mechanisms of blood clotting was still largely incomplete. In
fact, of the molecules involved in this process, only thrombin
and fibrinogen had already been identified, although not
characterized (1).
In 1865, the German anatomist Max Schultze published
the first precise and convincing description of platelets in a
journal he had recently founded, Archiv für mikroscopische
Anatomie (2), as part of a study mainly dedicated to white
blood cells. He called them “spherules”, smaller particles
than red blood cells that occasionally clump together and
can participate in the accumulation of fibrous material (3).
This way, he recognized platelets as a normal component
of blood (2,3).
Till and Mc Culloch are recognized for the study of the
hematopoietic system, its composition and function (1961)
(4,5). When a cell multiplies, differentiates, and initiates
maturation stages, in every single cell line, it originates
families characterized and differentiated among them, with
the objective of producing leukocytes, erythrocytes and
platelets. The latter are functional cells presented in blood as
cell population incapable of self-sustainability (6).
Platelets are small non-nucleated cells that are originated
by the division of the cytoplasm of the megakaryocytes in
• Form: ovoidea
• Without nucleus
• Central part: basófilo
• Peripheral part: basofilo
with less intensity
• Abundant: organelles
Source: Own elaboration based on the data obtained in the study.
Platelet cells present in blood have a normal concentration
between 150000 y 450000mm3 (11,12). Every day, the bone
marrow of a healthy adult produces 1x1011 platelets; this
quantity can increase 10 times in events of high demand
(13).
Platelets are not strictly “cells”; they are cytoplasmic
fragments with an average lifetime of seven to ten days to
accomplish certain functions in the organism (12,14); in
various radioactive processes a minimum survival of ten
days in the blood for platelets has been determined. This can
be observed in balance states or physiological homeostasis.
30% of the circulating platelets can be stored in the spleen,
if they are not consumed in a coagulation reaction; platelets
are used to be disintegrated in normal conditions in the
spleen (15). Platelet destruction is 35.000±4.300 platelets
µl/day, which is not relevant when compared with its
production (4).
These cells comprise cytoplasmic granules and a
membrane that contains pro coagulant molecules; it stands
out glycoproteins from the membrane, whose objective is to
permit the interactions platelet-platelet and platelet-vascular
endothelium (16).
Rev. Fac. Med. 2016 Vol. 64 No. 2: 351-63
Cytoplasmic concentration has two types of granules:
alpha granules, which contain hydrolases, cathepsins,
fibrinogen, fibronectin, factors V-VIII, platelet factor IV,
platelet-derived growth factor and transforming growth
353
factor-alpha(17), and the delta granules —dense—,
constituted by adenine nucleotides —ATP and ADP— (17),
ionized calcium (16), histamine, serotonin and adrenalin
(18) (Figure 1).
Figure 1. Platelet generalities such as a) composition, b) degradation, c) time/origin, d) production, e) motor activity and f) phagocytes.
Source: Own elaboration based on the data obtained in the study.
Every cell has a membrane, in the case of a platelet, it is
denominated periplatelet atmosphere and constitutes large quantities
of coagulant plasmatic factors, for instance: factor I, V, VII, XI and
XIII, antiplasmin, plasminogen activators and lgM (4,19).
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The Role of Platelet and its Interaction with Aspirin: 351-63
Among their adhesion and aggregation properties, the main
function of platelets is to participate in homeostatic processes
through the formation of homeostatic plugs that initially seal
vascular and micro vascular defects (20).
In addition, platelets can directly participate in some
processes mediated by lgE —destruction of parasites— by
means of the union of lgE through a specific surface receptor
(21,22) (Table 2).
derived messenger RNA (mRNA) and the translational
machinery needed for protein synthesis (24,25). Pre-mRNA
splicing, a typical nuclear function, has been detected in
the cytoplasm of platelets, and the platelet transcriptome
contains approximately 3000 to 6000 transcripts. Analysis
of the platelet proteome is more complex. After leaving the
bone marrow, platelets circulate for about ten days. Their
primary function is to stop hemorrhage after tissue trauma
and vascular injury (24).
Table 2. Immunological properties of platelets with the interaction
of the membrane receptors IgG, IgE and the glucoprotein 1 b (GP1b).
Thrombocytes and coagulation
Platelets and defense
Organism defense
Platelets have antiparasitic defenses since they
have surface-fixed immunoglobulins with the
property of adhering to certain parasites and
releasing H2O2, exerting a toxic action on the
specific parasites that can proceed in platelets.
Normal values
150.000-400.000/mm³ (In units SI:
150-400x109/L).
Membrane receptor
IgG
IgE
Glucoprotein
1 b (GP1b)
When the vascular injury occurs, a series of events happen,
which, in a good performance, lead to the reparation of the
affected part (26). This process begins with the vascular
endothelium injury, then, a vasoconstriction of short-duration
occurs, mediated by reflex neurogenic mechanisms and by
local release of vasoconstrictor agents such as endothelin
(18,27-29).
The endothelium irregularity that caused the injury
(3) exposes the subendothelial extracellular matrix,
concomitant with the vasoconstrictor factors (18,2729). This exposition of the extracellular matrix (ECM)
facilitates the platelet activation where the platelets are
small circulating discs and turn into plain plates that adhere
to the injury (30).
Source: Own elaboration based on the data obtained in the study.
Currently, histological techniques permit to observe the
colored areas of the bone marrow cells with the microscope,
where precursor cells can be identified as the ones with
the function of originating mature cells (4). Thus, platelets
have been proposed as a peripheral biomarker of the
functioning of the central serotonergic neurons due to the
shared similarities among the capture, storage and release
mechanisms of serotonin, since 5-HT2A receptors are present
in the membranes of both cell types. The serotonergic system
of platelets has four main components: A capture mechanism,
intracellular storage organelles, serotonergic receptors in
the plasmatic membrane and a mitochondrial enzyme for its
metabolism —monoamine oxidase MAO—. All the elements
have physiological and pathophysiological similarities (23).
Platelets in Primary Hemostasis
Platelets are produced by megakaryocytes as anucleate
cells that lack genomic DNA but contain megakaryocyte-
New Model of the Process of Coagulation of the Blood
At present a new model of the process of coagulation of
the blood has been proposed; it begins for the TF contact
to the bloodstream. TF is a transmembrane protein that
acts as a receptor and cofactor for factor VII (31,32) isn’t
constitutively expressed in endothelial cells but is present
in the membranes of cells around blood vessels, such as in
smooth muscle cells and fibroblasts. Therefore, the TF is
exposed to the bloodstream due to damage to the endothelium
and surrounding cells or by the activation of endothelial
cells or monocytes (31). It is quite known that the FVIIa /
TF complex activates not only factor X but also factor IX.
Studies indicate that this complex is essential to start in vivo
clotting (33) (Figure 2).
The current understanding of the hemostatic process
considers the interrelationship of physical, cellular and
biochemical processes in a series of stages or phases and
not as two pathways —intrinsic and extrinsic— as it was
believed (34).
Rev. Fac. Med. 2016 Vol. 64 No. 2: 351-63
355
Figure 2. New model of the process of coagulation of the blood.
This new model of hemostasis is able to explain some clinical aspects of hemostasis that the classical cascade model does not.
It gives a better understanding of the in vivo coagulation process and is more consistent with the clinical observations of several
coagulation disorders. Source: Own elaboration based on the data obtained in the study.
356
Initiation phase
It takes place when the cells that TF expresses in its
surface are exposed to components of the blood where the
injury is located (35,36). The TF, once bound to FVII in
blood, quickly activates it forming the FVIIa/TF complex,
which is responsible for the activation of small amounts of
FIX and FX (37,38). FXa associated with its cofactor, FVa,
forms a complex called prothrombinase on the surface of
cells that express TF. FV can be activated by FXa or noncoagulating proteases, resulting in FVa which is necessary
for the prothrombinase complex. This complex turns small
amounts of prothrombin —Factor II— into thrombin; these
amounts are too small to complete the fibrin clot formation
process (36,38,39).
Small amounts of thrombin are continuously produced
outside the vascular space, independent of vascular injury. (37)
Therefore, it is assumed that a little activity in the TF pathway
occurs all the time in the extravascular space. Coagulation
process only proceeds to the amplification phase when there
is vascular damage, i.e., when platelets and FVIII —linked to
von Willebrand factor— come into contact with extravascular
tissue where they adhere to cells expressing TF (33,40).
Amplification phase
Platelets and FVIII bound to von Willebrand factor
(vWF) only pass to the extravascular compartment when
there is a vascular injury. When a vessel is injured, platelets
leave the vessel, bind to collagen and other components of
the extracellular matrix at the site of injury, where they are
partially activated and form a platelet plug responsible for
primary hemostasis (41,42). At this point, small amounts of
thrombin produced by cells that express TF can interact with
platelets and FVIII/vWF complex. This starts the hemostatic
process that culminates in the formation of stable fibrin,
secondary hemostasis, which consolidates the initial platelet
plug (35,37,42).
This small amount of thrombin generated by cells
expressing TF has several important functions being the
main one the maximum activation of platelets, which make
receptors and binding sites available to activated coagulation
factors. As a result of this activation, the permeability of
platelet membranes is altered, allowing the entry of calcium
ions and the release of chemotactic substances that attract
clotting factors to their surface in addition to releasing partially
activated FV (35,38).
Another function of the thrombin formed during the
initiation phase is the activation of the FV and FVIII cofactors
The Role of Platelet and its Interaction with Aspirin: 351-63
on the surface of activated platelets. The FVIII/vWF complex
is dissociated, allowing vWF to mediate platelet adhesion and
aggregation at the site of injury. In addition, small amounts
of thrombin activate FXI (FXIa) on the platelet surface
during this phase. Activation of FXI by thrombin on the
platelet surface explains why FXII is not required for normal
hemostasis (36-38,40,43).
Propagation phase T2
The propagation phase is characterized by the migration
of large numbers of platelets to the site of injury and the
production of tenase complex and prothrombinase on the
surface of activated platelets (36). First, the FIX activated
during the initiation phase can now bind to FVIIIa on the
platelet surface forming the tenase complex. An extra amount
of FIXa can also be produced by FXIa bound to platelets.
As FXa can’t effectively move from cells expressing TF to
activated platelets, a large amount of FXa must be directly
generated on the platelet surface by the FIXa/FVIIIa complex
(40).
Finally, FXa rapidly associates with FVa bound to the
platelet during the amplification phase resulting in the
formation of the prothrombinase complex which turns large
amounts of prothrombin into thrombin. This is responsible for
the cleavage of fibrinogen into fibrin monomers that, in turn,
polymerize to consolidate the platelet plug (33).
Termination phase
Once a fibrin clot is formed at the lesion, the clotting
process must be limited to the injury site in order to prevent
thrombotic occlusion of the vessel. Four natural anticoagulants
are involved to control the spread of coagulation activation:
tissue factor pathway inhibitor (TFPI), protein C (PC), protein
S (PS) and antithrombin (AT) (34).
Thrombocytes and Inflammation
Every tissue is composed by barriers that permit protection
—membranes or set of cells—. These barriers can be affected
by microorganisms, infectious agents or antigens that enter
into the tissue through a physical injury; the barriers have to
face the inflammation responses (26), this occurs due to the
vascularization of living organisms. Therefore, the action of
physicochemical processes of this response leads to leukocytes
and plasma molecules to places where the tissue alteration has
been produced. The main effects consist of an increase of the
blood flow by local vasodilation of the capillary permeability
to large plasma molecules and leukocyte migration through
vascular endothelium towards the site of inflammation
Rev. Fac. Med. 2016 Vol. 64 No. 2: 351-63
(4,44,45). Cell migration takes place at the level of the venules,
while the exudation of plasma proteins is mainly produced at
the level of capillaries, where blood pressure is higher. This
cell migration permits the complement, antibodies and other
enzyme systems, to reach the site where the inflammatory
response occurs (27,44).
Thrombocyte can be an indicator of inflammation if there
are C-reactive protein elevations, fibrinogen and other acute
phase reactants. This indicator is associated to rheumatoid
arthritis. The presence of thrombocytes is correlated with
the activation of an inflammatory process, osteomyelitis,
rheumatic fever, ulcerative colitis, malignancies and acute
infections (9).
Platelets and Development of Atherosclerotic Lesions
Platelets that adhere to the vessel wall at sites of
endothelial-cell activation contribute to the development of
chronic atherosclerotic lesions, and when these lesions rupture
they trigger the acute onset of arterial thrombosis. Platelets
adhere to the endothelium of carotid arteries in apolipoprotein
E (apoE)−/− mice before atherosclerotic lesions are visible
(24). Von Willebrand factor, when secreted in large amounts
by endothelial cells in response to inflammatory stimuli,
can recruit platelets to the site; the interaction between
glycoprotein Ib and von Willebrand factor allows platelets to
roll on endothelial cells (24). The atherosclerotic lesions in
von Willebrand factor -/- mice are smaller than the lesions in
wild-type mice (24).
The acceleration of atherogenesis by COX-1-, the inhibition
of the synthesis of platelet thromboxane, as well as the
antagonism or deletion of the thromboxane receptor, delay
atherogenesis in murine models. Dependent thromboxane in
low-density lipoprotein (LDL)-receptor -/- mice suggests that
platelet activation increases the rate of plaque formation (24).
Activated platelets can also influence the progression
of plaque formation by releasing adhesive ligands, such as
P-selectin, that become expressed on the platelet membrane
and mediate platelet-endothelium interactions (24).
Signaling by P-selectin stimulates monocytes and
macrophages to produce chemoattractants or growth factors.
Moreover, engagement by P-selectin of the P-selectin
glycoprotein ligand 1 on the monocyte surface initiates the
formation of platelet-monocyte aggregates and outside-in
signaling that induces the transcription of COX-2 (24).
Prolonged adhesion-dependent signaling promotes the
expression of interleukin-1β. This cytokine enhances the
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stability of COX-2 mRNA, thereby promoting synthesis of
the enzyme. Activated platelets exacerbate atherosclerosis in
apoE-/- mice in a P-selectin- dependent manner (24).
Platelets in Tissue Repair
Besides mediating primary hemostasis and thrombosis,
platelets play a critical role in tissue repair and regeneration.
They regulate fundamental mechanisms involved in the
healing process, including cellular migration, proliferation,
and angiogenesis (46).
In addition to the fact that platelet-rich plasma has
increasingly gained attention to seal wounds and enhance
wound healing, (47) experimental and clinical data clearly
indicate that platelets are fundamentally involved in repair
and regeneration of damaged tissues and preservation of
organ function. During tissue injury, for example caused by
trauma or local ischemia as seen with myocardial infarction or
stroke, the coagulation system and immune responses become
activated very early, initiating the process of wound healing.
Platelets are the first cells that accumulate at sites of the
lesion and, on activation, release a multitude of biologically
active mediators into their microenvironment (48). Various
cytokines, chemokines, and growth factors, including
CXCL12 —stromal-derived growth factor 1, SDF-1—, (49)
and hepatocyte growth factor (HGF) (50), have been identified
to be secreted from platelets. Platelet-derived mediators
induce and modulate activation of fibroblasts and recruitment
of leukocytes, first neutrophils, followed by macrophages,
resulting in the elimination of dead cells and cellular debris
(51). Moreover, platelet-released factors induce and control
proliferation and migration of other cell types that are
critically involved in tissue repair such as smooth muscle cells
(SMCs) and mesenchymal stem cells (MSCs). Angiogenesis
in damaged tissue, another pivotal mechanism for recovery
of tissue function, is also substantially regulated by platelets
due to the release of a multitude of pro and antiangiogenic
mediators upon platelet activation (46).
Nowadays, platelets and their secretory products may
successfully be used as feasible therapeutic tools, facilitating
repair of injured tissues and organs. For instance, autologous
platelet releasate as well as recombinant platelet-derived
growth factors may enhance healing of chronic lower
extremity diabetic ulcers. Moreover, regeneration of cutaneous
wounds, retina, and peri-implant bone by platelets has been
reported. However, treatment of surgical lesions with plateletrich plasma has also generated controversial results in clinical
trials (52). Such new insights will help researchers to find
better therapeutic platelet-based options to facilitate repair
and regeneration of injured tissues and organs (46).
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The Role of Platelet and its Interaction with Aspirin: 351-63
Growth Factors in the Platelet
When speaking about injury, the thrombocyte that tackles a
degranulation process during the formation of the clot liberates
factors of growth (FC): platelet derived growth factor (PDGF),
vascular endothelial growth factor (VEGF), fibroblast growth
factor (FGF), insulin-like growth factor-1 (IGF-1), epidermal
growth factor (EGF) and transforming growth factor beta
(TGF-β); these are powerful chemotactic factors of inflammatory
cells, especially PDGF and the TGF-β (53,54) (Figure 3).
Figure 3. General scheme of the platelets by releasing growth factors that lead the repair. Source: Own elaboration based
on the data obtained in the study.
It seems that FC liberated from thrombocytes are the
initiators of the cascade of events that lead the repair, spreading
quickly from the place of the injury and are degraded for
proteases (54,32).
cells (MSCs) and other cellular types critically involved in
the healing of wounds (54).
They liberate these cytokine, chemokine and such growth
factors like stromal cell derived factor 1 (SDF-1) and
hepatocyte growth factor (HGF) that control the recruiting,
proliferation, and activation of fibroblasts, neutrophils,
monocytes, smooth muscle cells (SMCs), mesenchymal stem
Platelet-rich plasma (PRP) is used for tissue regeneration
in different clinical settings and offers several advantages
since it is neither toxic nor immunoreactive and can be readily
obtained, inexpensively, from the blood of patients themselves.
Various studies have been published on the preparation and
Platelet Rich Plasma
Rev. Fac. Med. 2016 Vol. 64 No. 2: 351-63
utilization of PRP for wound healing with controversial
results (55).
Marx et al. reported the combination of PRP with a mixture
containing 10% calcium chloride and bovine thrombin
favored jaw reconstruction. The utilization of PRP has yielded
satisfactory outcomes in dentistry and maxillofacial surgery,
plastic surgery, trauma and orthopedic surgery, tendon and
ligament repair, and ocular repair (55).
Aspirin and its Interaction with the Thrombocyte
There is historical background regarding the use of willow
leaves by Sumerians and Chinese people as analgesic more
than three thousand years ago. Long after, that tree would give
origin to aspirin. In the 5th Century BC, Hippocrates of Kos,
the father of medicine, by reacquiring ancient knowledge,
used the concoction extracted from the bark and leaves of the
willow Salix Latinium to relieve aches and to control fever.
During the Middle Age healers boiled willow bark and gave
this miraculous potion to people suffering from pain (56,57).
In 1835, the French chemist Charles Frederic Gerhardt tried
to surpass the problem of the salicylic acid as it was bitter
and caused trouble. Then, he combined sodium salicylate
with acetyl chloride obtaining acetylsalicylic acid. He did not
believe in the future of this compound and abandoned this
project. In 1859 Kolbe synthesized the acetylsalicylic acid
(ASA). In August 10th, 1897, Felix Hoffmann, as a worker at
Bayer’s laboratories, started a new era in pharmacology as
he discovered acetylsalicylic acid, aspirin active ingredient
(56,57).
In 1954, it was demonstrated that acetylsalicylic acid could
increase the bleeding time and, in 1970, that it could elucidate,
that is, it has an antithrombotic effect in animals. Based on its
hematologic actions, Vane demonstrated that it was antiplatelet
since it is an important prostaglandin inhibitor. Later, Majerus
and Roth were capable of characterizing the interaction
between aspirin and platelets. Then, they demonstrated that
low concentrations of aspirin acetylized irreversibly platelet
cyclooxygenase. This way, thromboxane A2 levels decrease
(57).
ASA is administered orally and it is rapidly absorbed
through the gastrointestinal tract; this absorption occurs by
passive diffusion of lipophilic non-ionized molecules, partly
in the stomach but much more in the duodenum. Then, it is
hydrolyzed to SA (salicylic acid) by carboxylesterases of
the intestinal wall and liver; approximately 50% of the dose
reaches the systemic circulation in an unaltered form. Its
fast hydrolysis to SA determines a short half-life of 15 to 20
359
minutes, although this depends on the several dosage forms.
Once in the blood, SA adheres significantly to plasma albumin.
ASA is widely distributed to all organs, in particular, kidney,
heart, lung and liver. Finally, it is excreted through urine by
processes such as glomerular filtration and tubular secretion
(58,59).
Aspirin, a medication widely used due to its antiinflammatory and analgesic properties, exerts its action by
inhibiting the production of prostaglandins, which are derived
from arachidonic acid (AA) (60-62).
To carry out this process, it is required an enzyme called
cyclooxygenase (COX), three isoforms of COX have been
described: COX-1 is present in almost all tissues, it catalyzes
the conversion of AA to thromboxane A2 (TXA2) and it
is agonist in the process of platelet aggregation; COX-2 is
produced in response to an inflammatory stimulus and COX-3
is located in the brain and the bone marrow and participates in
processes of neurological regulation of pain.(60-62) (Table 3).
Tabla 3. General effects of aspirin in the organism.
General Effects of Aspirin in the Organism
It inhibits the
production of
prostaglandins
It is a heart
protector
It reduces
preeclampsia
It stimulates the
increase of
cysteinyl-leukotrienes
Source: Own elaboration based on the data obtained in the study.
In addition, aspirin (ASA) allows to modify the prothrombotic platelet properties —platelet plug formation— to
block prostaglandin synthesis, to inhibit irreversibly the COX1
and to prevent platelet aggregation, considered as one of the
initial phase in the formation of blood clots. Due to the fact
that platelets have no nucleus, they do not have the capacity
of COX generation, therefore, there won’t be synthesis of
TXA2 since it depends on the bio-synthesis of new platelets
(60,61,63).
This medicine inhibits diacylglycerol production in
platelets, then, it can inhibit secondary aggregation induced
by thrombin, collagen or ADP. Thus, the effect is less lasting
and less dose-dependent. Furthermore, it prevents platelet
dense granule secretion. Salicylic acid, metabolite of aspirin,
has a certain fibrinolytic effect because of its interaction by
neutrophils and monocytes with the release of proteolytic
enzymes —Cathepsin G and elastase—. It is recognized its
effect against pain, inflammation and fever and as agonist its
platelet anti-aggregation effect by the reduction in the formation
of thromboxane A2 is taken into account, that is why it can be
used as a cardiac muscle protector (25), for its protective action
in patients who had suffered a myocardial infarction has been
360
proved (64) and its regular use in low doses has enabled them
to avoid the possibility of a second attack (25).
Aspirin or acetylsalicylic acid is, without a doubt, the
greatest discovery in the history of cardiovascular disease from
the point of view of prevention. However, there is not enough
clinical data that identify the most effective and safe dose.
For the management of cardiovascular disease, the aspirin
dose suggested lies between 75mg and 325mg daily. Some
studies have demonstrated that the daily aspirin dose of less
than 162 mg is associated with a low incidence of thrombotic
and bleeding events (65).
The steering committee of a physicians health study
research group observed that the main benefit of aspirin in
preventing myocardial infarction was present exclusively in
people possessing high levels of C-reactive protein, suggesting
as a clinical advantage of aspirin that can be derived, in part,
from its anti-inflammatory effects (12).
Preeclampsia is not just hypertension induced by pregnancy,
but there also is an interaction between reduced placental
perfusion and alteration in the maternal endothelial function
(66). In addition, some recent evidence has found mild to
moderate benefits in the use of low-dose aspirin to prevent
preeclampsia in patients at high risk for this type of medical
complication. This is due to the fact that low-dose aspirin
intake generates high concentrations of aspirin in the portal
circulation, which inhibits selectively the cyclooxygenase of
the platelets passing through the intestinal capillaries, while
concentrations in the peripheral circulation remain very low
to inhibit endothelial cyclooxygenase (67).
Finally, aspirin is a non-expensive medicine that does not
require monitoring. It has multiple therapeutic actions that
have been proven over more than a century of history such
as analgesic, anti-inflammatory, antipyretic and vascular
protector. Moreover, ASA has demonstrated to be a highly
effective, safe and reliable medicine (59,68).
The Role of Platelet and its Interaction with Aspirin: 351-63
flowing blood, thereby initiating the formation of a thrombus.
Exposed collagen triggers the accumulation and activation of
platelets, whereas exposed tissue factor initiates the generation
of thrombin, which not only transforms fibrinogen into fibrin
but also activates platelets (69).
Low Dose Aspirin for Preventing Recurrent Venous
Thromboembolism
Patients who have had a first episode of unprovoked
venous thromboembolism are at high risk for recurrence after
anticoagulant therapy is discontinued. Long-term treatment
with a vitamin K antagonist is very effective in preventing
a recurrence of venous thromboembolism while treatment
continues but it has not been proven to improve survival;
furthermore, it is associated with a substantially increased risk
of bleeding and is inconvenient for patients (70). Consequently,
many patients who have had a first episode of unprovoked
venous thromboembolism discontinue anticoagulant therapy
after 3 to 6 months despite recommendations to continue on
therapy (69,70).
Low-dose aspirin is a simple, inexpensive, and widely
available treatment effective in the prevention of arterial
vascular events and the primary prevention of venous
thromboembolism in high-risk surgical patients (70).
Aspirin may also be effective in preventing a recurrence of
venous thromboembolism after a first event. The objective of
this review was to evaluate the efficacy of low-dose aspirin,
compared to placebo, in preventing a recurrence of venous
thromboembolism in patients who had completed initial
anticoagulation with warfarin after a first unprovoked episode
of venous thromboembolism (70).
Aspirin and Gender Differences
The vessel wall, with its inner lining of endothelium,
is crucial to the maintenance of a patent vasculature. The
endothelium contains three thromboregulators: nitric oxide,
prostacyclin, and the ectonucleotidase CD39, which together
provide a defense against thrombus formation (69). Collagen
in the subendothelial matrix and tissue factor facilitate the
maintenance of a closed circulatory system (69).
There are very well known differences in the epidemiology
of vascular events in men and women. Men are more prone to
stroke and myocardial infarction (MI), yet women are more
likely to die from these events (71). Similarly, meta-analysis
results suggest that there are also gender differences in the
effects of aspirin on cardiovascular disease (CVD), whereby
risk of MI appears to be reduced in men and risk of stroke
in women (72). Current guidelines for the use of aspirin for
CVD prevention take these differences into consideration.
Aspirin use in males is primarily intended for the prevention
of coronary artery disease, while in females, prevention of
stroke is the main target (73).
When the vessel wall is breached or the endothelium is
disrupted, collagen and tissue factor become exposed to the
The reason for the aspirin therapy effect differences in
terms of gender is currently unknown, but evidence suggests
Formation of a Platelet Thrombus
Rev. Fac. Med. 2016 Vol. 64 No. 2: 351-63
that there may be some biological basis for these differences.
For example, baseline platelet reactivity is greater in women
than in men, with higher residual reactivity following aspirin
treatment in women (74). As such, physicians should also be
sure to consider gender-specific risks, benefits, and guidelines
for aspirin therapy prior to making patient recommendations
(73,74).
Conclusion
Aspirin inhibits platelets functionalism, preventing their
aggregation and thrombus formation within the spleen and
other bodies. Thrombosis plays an important role in the
manifestations of cardiovascular diseases, which are the main
cause of morbidity and mortality worldwide.
The benefit of low-dose aspirin therapy for the secondary
prevention of serious cardiovascular events is clear, but the
use of aspirin for the primary prevention of cardiovascular
disease (CVD) remains a challenge due to mixed findings on
mortality benefit.
Conflicts of interests
None stated by the authors
Financing
None stated by the authors.
Acknowledgements
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